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Does electrical usage contribute to fuel consumption?
I know that while in-flight the electricity is produced by the engines.
What happens if every possible system requiring electricity is in the on state? This should require extra electricity.
Does this usage of electricity increase the fuel consumption by any means?

*Edit to include the additional question from the comments section:
Given that the engines require fuel to produce electricity how big is the impact on fuel consumption, speaking of "mainstream" airliners like the A320?

If you have ever driven a car in a tropical region during summer, you'd notice that with the air-con turned on, fuel consumption per mile goes WAY up.

You might get better answers if you specify large transport AC or GA aircraft.  I suspect the answer might be very different between a 737 and a C172.

@kevin Its not what you think. The fuel consuption of a car increases while the A/C is on is because the compressor is connected directly to the engine. Info from our sister site.

Short answer
In flight, unless you use external energy sources like the Sun, each change in the use of electricity leads to a proportional change in the quantity of fuel burnt in the engines (or in the APU).
On an aircraft, generators only convert fuel potential chemical energy into something else, usually electricity, hydraulic energy or pneumatic energy.
There is no other source of energy:

The batteries themselves need to be charged, charging burns fuel.
The RAT (ram air turbine) uses energy from the relative wind created by the engines thrust. If the RAT is used because the engines and the APU are down, the relative wind is due to the loss of altitude created by the gravity. This kinetic energy is the restitution of the potential energy accumulated during the climb with the engines, so with fuel.

Details
Maximum power of the electrical equipment
The electrical power needs of the whole aircraft can be completely met by a single engine generator, or by the APU. On the A320 family, the APU is an APS3200 delivering about 90 kVA.
APU fuel consumption
From this PPRuNE discussion, an A320 APU may consume 130 kg per hour on the ground, and 51 kg per hour at FL300. However the APU provides at the same time electricity and bleed air energy.
APU fuel consumption for electrical energy generation
However the fuel quantity used for the sole electricity generation may be approximated:

Maximum power required by the aircraft equipment → 90 kVA AC.
Assuming a mean cos φ = 0.8 → Apparent power = 110 kW DC.
Assuming 25% efficiency for the system (gas turbine + generator) → Fuel power required 440 kW.
Fuel energy required → 440 kWh per hour.
Converting to joules (1 MJ = 0.28 kWh) → 440 kWh ≈ 1,570 MJ.
Kerosene specific energy = 43 MJ/kg → Fuel quantity burnt in an hour = 1,570 / 43 = 36 kg (45 L)

So approximately 36 kg of fuel are required per hour to provide for electric energy on an A320.
Note that in flight all engines generators will be active and will share the total demand (feeding different buses for safety), and the APU will likely be inactive. The fuel consumption should be more or less the same, regardless of the number and nature of the active generators. An inactive or unloaded engine generator can be seen as not impacting the fuel consumption for practical purposes.
Engines burning the maximum rate of fuel
Engines have a maximal power. When the engines are already at full thrust, increasing the electric load will divert an additional amount of energy to the generator. This quantity being removed from the energy usable for aircraft propulsion, the aircraft will slow down.
(Updated, taking into account the multiple helpful comments -- Thanks!)

Bonus
Breathing increases fuel use too:

Each time a passenger breathes, they reject heat and CO2 which must be evacuated out of the aircraft.
Some quantity of air must be replaced by new air by the conditioning system.
This system uses pneumatic or electric energy, and again fuel must be burnt to create this energy.

Any ideas how big the impact is? Ideally speaking of "mainstream" aircraft like the A320, 737 or such.

The engine has the limited efficiency. It probably needs to burn at least twice as much fuel to produce that amount of energy.

The efficiency of a jet turbine is actually about 40-45%. Everything you want to know about APUs is here: http://www.b737.org.uk/apu.htm

737 APU under full load uses 240lb/hr (130l/hr) to generate 90VA. so you're wrong by factor of 13 : D
http://aviation.stackexchange.com/questions/8429/does-the-apu-consume-a-lot-of-fuel-compared-to-a-jet-engine?rq=1

@PaulSmith That's pretty amazingly efficient.  Modern ground-based powerplants that generate electricity for profit tend not to get that high!

@Agent_L: Your source says 90 kVA, not 90 VA -- a significant difference. (And it's strange to me why it doesn't just say 90 kW).

@HenningMakholm: VA and W are different units, due to the exisence of a phase difference between current and voltage in a non purely resistive circuit in AC. This involves reactive power and the complex number set.

You don't mention the difference between a fully loaded generator, and the more typical case where all redundant generators are sharing the load (more specifically he asks if you turn something additional on, does it matter, or is the generator using a fixed amount of fuel anyway).  Also, as part of your bonus answer, this "wasted" power can be recaptured in some cases if needed (turn the bleeds off for more thrust) and is significant enough to be part of the procedures developed by the manufacturer  (takeoff and two of three engines inoperative on the aircraft that I fly).

@Lnafziger: That's right, the APU won't probably be used in cruise, and the engines generators will feed different buses. But overall the max power required won't significantly change, the fuel consumption may be a little bit greater (I've updated the answer to clarify, thanks!). The bonus was for fun and food for thought. Still, when the packs are off, no energy is tapped from the engine, heat accumulates in the cabin, but is not reused per se. Or is it in some designs?

@mins: The reactive load is not something the generator needs to "produce", though. It ought to lead to no average torque on its drive shaft.

@HenningMakholm Yes, the very reason that electricians worry so much about the power factor is that the generator HAS to produce this power. Power that can't be used anywhere but is wasted in the wires of the entire system.

@Agent_L: Shouldn't power that is wasted in the wires be ohmic load? Otherwise there's no net energy loss over a cycle.

@HenningMakholm "Waste" power (current to be more precise) has to be sent to the receiver, but the receiver doesn't do useful work with it, because voltage isn't there at the moment. But the current flows, so yeah, it is lost as ohmic load in the wires. That's the problem - wires act as the ohmic load instead of the intended receiver. The system as a whole of course can't lose any energy. But it goes into ohmic heating of the wires instead of useful work. Or, to be more precise - into more ohmic heating of the wires than it would happen with optimal power factor of 1.

@mins Well, if it doesn't tap energy from the engine, then it is available to be used elsewhere (more thrust). That's the whole point of a "bleeds off takeoff".

@Phantomazi the first result I found quoted 2500 kg/h flying fuel consumption for a 737-800, so the APU would be using about 1.5% of it max. Or the Amdahl corrolary: no reduction in electrical use can make the plane more than about 1% more efficient.

It's even true in a car. Your car gets less gas milage with the headlights on than off.

Yes, because (like a car for example), the aircraft is a closed system, and all energy must be provided internally.
So, in flight, that energy must come from the engines, and therefore, the engine must either slow down, or use more fuel.

This is a fantastic answer, as this explains the reason in such simple terms. +1

Welcome to the site, user14035! This is a nice little answer that hits the heart of the matter. If you'd like to stick around, I suggest registering your account so you can pick a more memorable name. I can't remember if you can register an existing account or if you need to create a new one; if you do need to create a new one, contact the site admins to get the two accounts merged.

And this is exactly why your car has higher fuel consumption when you run the aircon.

@DavidRicherby, you're not wrong, but I wouldn't say "exactly" because the A/C on a car is a much more of a direct mechanical drain on the engine rather than an electric drain.

@JPhi1618 Wikipedia says that the cooling compressor of car aircon systems can be powered either electrically or by a belt from the engine. I agree that, in the latter case, it's a direct mechanical drain; I'd assumed it was always electrical.

@DavidRicherby, Interesting.  I've never seen an electric compressor on a car.  Now I really want to know if that exists outside of a electric or hybrid car.

Is there not excess energy from the engines that could be put to use generating electricity without requiring any more fuel? It doesn't seem like too hard of a problem.

@Mohair, no not really.  The electrical generator under load acts like a brake on the shaft.  If that brake is released, the engine speeds up and produces more thrust.  There's no energy "sitting around" waiting for something to use it.

@Mohair Suppose the engines were producing more energy than they needed to. Then that would be a waste of fuel, so they'd reduce fuel flow to the engines until the engines weren't producing more energy than they needed to.

@immibis Show me an engine that doesn't produce more energy than it needs. There's always an excess. Put some magnets on the shaft, surround them with coils and you have a generator. The energy drain that would cause is probably irrelevant.

@Mohair Do you understand that the current flowing in the coils produces a magnetic force that slows down the spinning magnets? (If you have nothing connected to the generator, you get no current and no opposing force, but you're not producing electricity either)

@Mohair The "excess" energy released by an engine goes into vibrations and heat loss. If there were a currently-feasible way to capture this (i.e. one that cost less than the fuel savings,) we'd already be using it. The engines on an airplane are not producing significant excess thrust... if they were the plane would climb and/or accelerate.

You can never have an excess of energy, all energy produced must be consumed either as useful work (moving the vehicle) or wasted work (vibration and heat loss). If you put more in than you get out the difference will accelerate the vehicle until air resistance etc increases to the point that the system is still in balance.

@Mohair: They use pretty much exactly that procedure for eddy current brakes...

@Mohair, think of this comparison: You can run with a rucksack containing 30 kg of stones. You currently run with 10 kg, you burn a n calories. Your load is increased to 30 kg. Because of this additional load: 1/ you burn more calories and oxygen per minute; 2/ you will be exhausted more quickly; 3/ you won't be able to run on the same distance. While your power is 30 kg, the work you deliver is your actual load multiplied by the distance it is carried on. Energy used is not proportional to the available power, but to the work actually done.

It is just not possible otherwise but the effect should be somewhat below 10 % even when maximal amounts of electricity are consumed. Much more power is required and consumed just to fly.
A generator axis must rotate in order to produce electricity, and the resistance to rotate is proportional to the generator load (when the generator is not loaded, only friction remains). That's physics.
An engine that is coupled to the generator will meet more resistance and should consume more fuel when powerful energy consumers are connected to the generator.
However if the A320 generator requires 90 kW (as from another question), it does not make significant part of the engine power. An engine of 747 produces 16275 Kw and even turboprop engines are rated of the order of a few thousand kW (3,362 kW for C-130 here), so the generator is unlikely to have significant impact on the power produced even when running at the full load.


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Is it possible to create a direct supersonic airflow to a turbofan's afterburner in order to enhance the performance or reduce the fuel consumption?
so I found out that SR-71 Blackbird is using what's called "turboramjet" and I find the idea a bit appealing since they say with such engine, the Blackbird is more fuel efficient at it's top speed. I know that the mechanism for such engine is extremely complicated, but what's on my mind is not making an engine like Blackbird's, more to the point of having a direct supersonic fresh airflow to the afterburner to keep the fuel consumption slightly lower thanks to more oxygen to keep the afterburner effectively burn(because I heard somehwhere afterburners burns 3 times more fuel than dry thrust, I hope this can reduce it to only twice or even 1.5 times original dry thrust fuel consumption and have the same performance). Is it possible to do such thing? what would be the main problem to such engine/design? can it produce more thrust/more efficient in fuel consumption? and for the sake of calculations, let's say the turbofan engine in question is GE F-414-EPE, on afterburner, in supersonic speed.

Are you asking whether the turbofan you mention could be modified to become a ramjet (or well, ram-fan)? There's a lot of fluff to your question which makes it hard to condense your main point.

not necessarily a ramjet, more on to make a direct fresh airflow to enhance the combustion process in afterburner, the jet engine is still running during that time unlike Blackbird's turboramjet

A turbofan already has bypass air, which could be considered 'fresh' air for the afterburner. So I'm not sure what else you're looking for.

you're right, I might have to edit the question a lot...
I'm was looking for a way to put direct supersonic airflow into the afterburner, since turbofan's bypass air is not really supersonic, I'd like to see if I can add more air for that

Burning fuel in super sonic flow is hard. It's in active research but hasn't made into any service aircraft yet. Also your question makes it a much much harder because it implicitly requires the turbine exhaust be super sonic, and potentially also super sonic in the main combustion chamber. Most jet engines, including turbofan, turbo jet, or rocket engine doesn't have any super sonic flow until the nozzle. If you want supwersonic flow from the intake, through compressor, combustion chamber, turbine, into afterburner, plus supersonic bypass. I think 20 years of R&D from now is quite optimistic.

The J-58 took compressed air from the compressor at stage 4 and piped it directly into the flow aft of the turbine. This cooled the exhaust stream that entered the afterburner, so the starting temperature there was lower and the density higher, which increases efficiency and thrust. Note that the intake of the SR-71 slowed the flow down to Mach 0.4, so all internal flow was subsonic. Only when the hot exhaust gas in the afterburner expanded again would the flow speed increase to supersonic speeds again.
This is only possible because the intake would already compress the air by a factor of nearly 40 when flying at Mach 3.2. This precompression scales with $$p_0 = p_{\infty}\cdot\frac{(1.2\cdot Ma^2)^{3.5}}{\left(1+\frac{5}{6}\cdot(Ma^2-1)\right)^{2.5}}$$
so the precompression is much lower (less than 6) for a typical F-414 maximum speed of Mach 1.8.
($Ma$ = Mach number, $p_0$ = ram pressure, $p_{\infty}$ = atmospheric pressure).
The gains in efficiency scale with the ratio of starting and exhaust temperatures (measured from absolute zero), so the increase in efficiency is much lower than what you seem to hope for.
Make sure that flow speed at the entry of the afterburner is decidedly subsonic. Supersonic flow at the start of the afterburner section will only delay ignition until the flow exits through the nozzle. All afterburners have rings inside, called flameholders, which cause local separated flow, so some burning gas is always present to ignite the newly arriving fuel-air mixture.

hm, so even if theoretically the engine goes at Mach 2, it only reaches 3... well, it was a cool idea, thanks for answering Peter!
wait... the Ma is for the Mach numbers right?

@AlbertG.: Right!

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What is the piston/leg on the aft wall of the L-1011?
In this response to the APU location on an L-1011, in this image, there appears to be a large piston or leg extended downward along the centerline from the aft wall:
Modified: By Reedy (Own work) [CC BY-SA 3.0], via Wikimedia Commons
Question is, what is that and what is its purpose? Silly guesses: A giant locking pin? A shock absorber?
Definitely does not seem like an effective tail-strike device. Also does not seem to be identified in the cutaway

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Taxiway alignment on scissor intersecting runways
I came across this announcement on the first flight on Dublin Airport's new runway and was struck by something in the photo I had not seen before.
The traditionally parallel taxiway makes a dogleg at the scissor intersecting runway such that the taxiway intersection is at 90 degrees. It intuitively certainly seems like a much safer design in general and than some of the other EIDW intersections which are explicitly called out  in the Airport Diagram (below) as a Hot Spot for Runway Incursions.
A quick search for "taxiway cross-runway design" had many results, including references to "Problematic Taxiway Geometry (PTG)", in FAA presentation, "FAA Office of Airport Safety and Standards, Pan American Regional Aviation, Safety Team (PA-RAST), Airport and Runway Safety and Technology, which states "Aligned taxiway (Prohibited)" page 8, and FAA presentation "Runway Safety Initiatives", page 16, etc.
When was the "Aligned Taxiway" prohibited? What Human Factors led to that design (ie: improved visibility from the cockpit, enhanced situation awareness, procedural clarity, etc.)? References to decision backing material appreciated.
The scissor intersecting runway configuration is very common in the USA and globally, even at tiny airports, like MINOT (MOT), N. Dakota, more so than the "V" alignment for a second runway.
Does this only apply to new construction or is there an expectation of retrofitting existing airports (eg: KBOS, where space allows)? Are there other operational examples in place? What is the experience of any pilots navigating such a configuration? Does this lead to a new risk of pilots proceeding straight at the dogleg into the grass? Or worse if pilot mistakes taxiway for runway on approach ?

(image retrieved from article and resized)

(image retrieved from opennav.com)

I suspect this taxiway configuration for the new construction was done, not so much because older designs are prohibited or frowned upon, but to accommodate the existing airport infrastructure as much as possible while reducing runway incursion events.  The new airport taxi way joins up with golf taxi way to runway 16. A straight away taxiway could work here, but also requires a relocation of the ILS and DME equipment for runway 16 to a new position on the field.  A small dog leg like that would provide ease of taxiing to cross runway 16 at a minimal increase in cost.


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Do Airbus pilots have to practice flying in alternate law?
I am sure when getting their type rating for Airbus FBW aircraft, pilots learn how to fly in alternate and direct law. After that, how much opportunity do they get to practice this skill?
On Air France 447 and Indonesia AirAsia flight 8501, the A/P switched off unexpectedly and the control law degraded to alternate 2. It appears in both situations that the PF had trouble stabilizing the aircraft roll attitude. It's difficult to determine whether this contributed to the pitch up and eventual stall but the original pitch up command may have been inadvertent while struggling to level the wings.
Under normal law, which pilots are accustomed to flying, the FAC calculates the amount of aileron deflection required to turn the amount of lateral sidestick deflection into a rate of roll with a maximum of 15° per second. In alternate 2 law the computer cannot make these calculations so it becomes direct stick-to-aileron control.
This can mean that the aircraft responds very differently than what the pilot is used to. Sometimes the difference may be slight but at high speed and high altitude the same stick deflections can result in a significantly higher roll rate.
Additionally the stick does not provide tactile feedback as to how much force is required to move the control surfaces. With conventional flight controls "roll or pitch rate would more directly correlate with the force applied on the control wheel, and not the amount of deflection required."1
These things combined can result in the pilot over-controlling. The control inputs can get out of phase with aircraft movements resulting in pilot induced oscillation in the roll axis. This occurred in both of the above accidents.
It must be terribly disconcerting for a pilot when the A/P disconnects and they are handed control of an aircraft they fly every day, but doesn't respond the way they are used to. I'm wondering how much practice they get at flying in this configuration, either in a simulator or in the aircraft itself. Do most airlines flying FBW aircraft require this regularly? Does Airbus - or anyone else - recommend any amount of further training or practice? Have these recommendations changed in light of the accidents?

1Bill Palmer, Understanding Air France 447

The Roll wasn't the important part. The Pitch was. In both these accidents the problem wasn't an inability to control the aircraft, the problem was that one pilot in each case tried to pitch up (incorrectly) which cancelled out the other pilot trying to pitch down and prevented stall recovery. Without this, both flights would likely have recovered fine.

@Jon Story I disagree that the problem wasn't in part inability to control the aircraft. An a/p disconnect in cruise should be a non-issue. All that was required was to grab the stick and maintain level flight. Neither of these two pilots was able to do that. In the process of trying to right the plane with suddenly unfamiliar controls both pilots introduced the pitch up that cascaded through a series of other errors into tragedy.

@TomMcW "All that was required was to grab the stick and maintain level flight". It's easier to say, when you already know the pitot is obstructed intermittently, and you have to ignore the related indications. So I'd rephrase your statement as *a part of the solution was to understand the speed indicator was unreliable, but the artificial horizon was correct, and apply the procedure for unreliable speed indicator".

@mins Of course it's easy to say in hindsight. I'm not pilot bashing, I'm saying the complexity of the aircraft worked against him. First order of business is to fly the plane but when he tried to do that he found it didn't fly like he was used to. He was giving full left and right stick movements to counter the roll oscillation even while the PNF was telling him to go light on it. They were in trouble before they even had time to do any troubleshooting. After the A/P disconnected it was only 5 seconds before he set off the stall warning and by 52 seconds they were in a full stall. cont...

...There's no indication he doubted the artificial horizon, he was just trying to right the plane but was struggling with unfamiliar controls. After they got into the stall certainly there was confusion and bad decisions, but had he been familiar enough with flying in alt law there would probably have been no upset at all.

@TomMcW: The roll control may have been made difficult by the transfer function of the alternate law which the crew was not enough trained for. Agreed. The altimeter and FD false indications could alone explain the stall, their role may have been critical. The crew had not identified the pitot  problem and the ECAM was not really helping in this identification. As mentioned in Wikipedia, no clear message stating the cause of the AP disconnection was offered to the pilots. They likely continued to use data that had been discarded as unreliable by the computers.

Just wishing to clarify that alternate law itself doesn’t necessarily feel different to normal law, it’s just that some protections are lost. If you’re in the middle of the envelope, that’s still ok. It’s direct law which feels different (and of course AF447 went iirc into Alternate Law 2 which is still load factor command in pitch but direct in roll). In my insignificant opinion, by the way, up until MCAS surfaced I felt alternate law without speed stability to be the most dangerous thing ever to get through certification on a post-1980 airliner...

@CptReynolds It’s the very fact of load factor in pitch combined with direct in roll that I believe precipitated the accident. In normal law you command roll rate which will be very different from direct aileron control. The latter will be much more conducive to roll oscillations as happened in this accident. Whereas the pitch autotrim still functioned to negate the natural pitch stability of a trimmed aircraft. Any inadvertent nose up command while fighting roll oscillations will be maintained by the computer. Unless you input nose down they will continue to build up until stall.

@TomMcW Yes I absolutely agree with that. Just nitpicking on terminology, that’s all...

It appears in both situations that the PF had trouble stabilizing the aircraft roll attitude.

Note that roll control is secondary when recovering from a stall.

Restoring normal pitch and roll, he stresses, are "of secondary importance"

The situation required that the pilots heed the stall warnings, be aware of the stall and put the nose down to trade altitude for airspeed.

Have these recommendations changed in light of the accidents?

You can read the recommendations in the official accident reports. These usually explicitly state what such changes should be. I believe it is very rare (except when investigators disagree) for regulators, manufacturers and airlines to ignore any such recommendations.

AF447
QZ8501

From what I recall, the major problems in these two incidents were not that the pilots were simply unfamiliar with how control inputs affect flight in alternate law. major problems were that the pilots did not seem to be aware that they were in alternate law or even aware that the aircraft was stalled. In both cases the pilots were confused, failed to follow procedures and failed to coordinate their actions effectively (i.e. CRM failures)
Airbus Flight Crew Training Manuals do cover alternate law.

Recommendations included

5.3 Aircraft Manufacturer
  1. The KNKT recommends that Airbus to consider in developing a means for
  flight crews to effectively manage multiple and repetitive Master Caution
  alarms to reduce distraction.
  2. The KNKT recommends that Airbus to consider and review the FCTM
  concerning the Standard Call-Outs in all phases of flight.

The Indonesian report lists  in some detail the actions taken by the airline and includes an Airbus article "What is a stall? How a pilot should react in front of a stall situation". It also includes some guidance issued by Airbus which includes recommendations concerning upset training in various control laws

After that how much opportunity do they get to practice this skill?

This depends on airline (and relevant regulators) more than on manufacturer. Upset recovery is probably something that all pilots should be trained in, and maintain familiarity with, regardless of who manufactured the aircraft.

I understand that the roll was secondary to the accident but the roll was the first upset and it was during the process of controlling it that both pilots inadvertently induced the pitch up. Your last section answers my question, though. Was the paragraph in the image included in the Indonesian report or was it issued by Airbus separately?

@TomMcW: The image is from the Indonesian report. It is also part of a separate Airbus document.

good answer and I feel adding that the problem of too much "autopilot" time (understandably, an A/P consumes way less fuel than a human pilot) is well known and discussed in the industry, to the point that a few airlines were discussing (idk how it ended up) about providing free/mandatory flying hours in gliders and GA planes to keep stick and rudders skill sharp. After all roll rates of a K13 aren`t that different from those of a 747 :p

Air France 477 was in a fully developed stall. In a normal flight attitude. All the way from 40,000 feet to the surface of the ocean. The aircraft was falling like a leaf from a tree, with an angle of attack of 90 degrees. No buffeting. Yes they rolled back and forth, again like a leaf in the wind.
Before AF 477, stall training focused on recognising the onset to stall, and how to prevent getting into one. The F/O who got the plane in a stall in the first place was partially guided there by the flight director: the BAE found in it accident report that Airbus avionics did not present vital flight information that the computers were aware of. Direct AoA indication to the pilots may have saved them, because they never realised they were in a fully developed stall. They were startled, alarms were going off, and they could not find a visual clue in their instruments what was happening.
Pitching the stick down would have initiated recovery. But the air data computer stops sounding the stall warning  at AoA over 70 degrees because that can't make sense, right?  at under 60 knots of forward speed. When they lowered the nose, the airflow came within the awareness region again and the alarm sounded. Pulling the stick back stopped the alarm sound. What would you have done, had you been the pilot?
They fell all the way into the ocean, not being aware that they were in a fully developed stall, because that was not incorporated in their flight training. Or in anyone elses - it has only recently been added to the training requirements. Alaska Airlines is one of the first to have added this facility to their simulator.
Your question is a valid one, but is based on only a small part of the chain of events that led to the crash:

Blocked pitot. Automation detects this, switches off, and sounds a tone. But where was the clear and immediate indication that the pitot data was invalid?
Confusing information on the Flight Director
Flying in alternate law, and therefore being able to actually enter a fully developed stall. The basis of your question.
Not recognising that they had entered a fully developed stall at cruise speed, cruise altitude. Stall prevention training had always focused on low airspeeds, not on Coffins Corner in the flight envelope.
Once they had entered the stall: no visual indication, only many sounds that there was something wrong. Pilot training 101: push the stick forward to recover from a stall. These guys had received those instructions during training in the little Cessnas! They knew that.

The captain was asleep when this happened, and locked out of the cockpit. He could have saved the situation, who knows? One of the blessings of middle age is less fight-or-flight and more of a clear head. Or just a fresh pair of eyes coming in and assessing the situation anew.
The Air Asia captain did realise they were in a fully developed stall, and immediately pitched the stick forward. He was an ex military pilot. The issue there was:

Language. An Indonesian national using English to instruct the French co-pilot to Push Back.
No direct force cue that the copilot was pitching back while the captain was correctly pitching forward. Airbus averages out conflicting stick inputs if no pilot has taken exclusive control by pressing the override button > 20 seconds.

Recommendations were made for Airbus to change flight information presentation.  From Cockpit Standard 1.7 onwards, direct AoA information is presented when the automation switches off.  See below under Update. Recovery from a fully developed stall will be part of the curriculum soon, and this is a challenge by itself. You've pointed the nose down, and you're diving straight down at mach 1. Please now gently pull the nose back up while keeping lower than N = 2.5 so that the wings don't snap off.
Update
Asked the question to an A380 pilot who upgraded from A330/A340 recently. According to him:

Training now includes flying in alternate law and basic law in multiple flight states, including cruise.
Proper procedure in the Air Asia crash would have been to command the F/O to not touch the stick.
The direct AoA I mentioned above is actually a backup speed tape which seems to work (is my understanding) on basis of AoA. Keep the needle in the green by providing pitch stick input.

I saw the YouTube video. My understanding is they were in a rather severe pitchup. I would have gone to full throttle and levelled the nose. I think even in a full stall that will get the horizontal speed up to something the avionics could understand, and quite possibly that is enough to recover.

When I did my initial type rating on the Airbus it was rare to find the aircraft in normal law. The thing is most of the complex failures such as dual hydraulic failures, emergency electrical configuration, multiple ADR failures, dual radio altimeters etc always has an effect of downgrading aircraft control laws. The first time I flew in alternate law was in FFS (Full flight simulator) session 3. This is where the instructor forces the aircraft into alternate law to show you how it works. So, for 2 hours you fly  in alternate law (no autopilot or flight director). It is pure raw data flying, doing ILS approaches and visual approaches. The whole point of the session is to get familiar with the reconfiguration control laws.
The sessions that followed also contained many scenarios where the aircraft went into alternate law. As I said complex failures always equal flying in alternate law and in the A320 the aircraft will always reconfigure to direct law when the gear is down (if in already in alternate law). That is the story of the one and a half month long conversion course. So, how about after that? As airline pilots we go back to simulator training once every 6 months so it is very frequent. My very first recurrent after the initial did include a complex failure. The dual radio altimeter failure. In this failure the aircraft goes into direct law once the gear is down and you have to fly a raw data approach in instrument conditions. So, do we fly alternate law every 6 months? The answer to that is no. We have so many failures that we need to practice so we cannot ram in failures that make the aircraft go into alternate law every time we come for recurrent. But the training is so frequent that it hardly poses a problem. There will always be alternate law here and there. And we do upset training which is now compulsory. It is always done in alternate law.

Here are Airbus approved A320 type program FFS 5 and FFS 6. I have marked in red those failures that put aircraft in a reconfiguration law.
I also want to comment on one of your statements.

This can mean that the aircraft responds very differently than what
the pilot is used to. Sometimes the difference may be slight but at
high speed and high altitude the same stick deflections can result in
a significantly higher roll rate.

Before I came to Airbus I flew in my airline's regional operations. I flew Dash 8s into small airports with short runways with little to know approach guidance facilities. So, we did a lot of visual approaches and a lot of hand flying. For me, personally, flying the Airbus is the same. I have never felt anything that different. And flying in alternate law is a non event. I have not found the response of the aircraft any difficult to handle. You should not make a statement based on what is written. You have to have first hand experience before you say something like that. One of the most important skills of a pilot is his ability to adapt. I mean in the Dash 8s, sometimes the control cable rigging makes aircraft controls heavy and sometimes weird which makes each air frame unique. But as a pilot you handle the situation and fly the aircraft. Flying in alternate law should not be a problem for a trained pilot. And we cannot really use one accident or actions of one set of pilots and use it to define the whole piloting community.

Thanks for your response. I'm glad you hear that you get upset training. I think that might have been made compulsory in response to the two accidents I mentioned in my question. There are many questions that are probably unanswerable in the AF447 crash. But the one I keep coming back to is this: why could a trained pilot not seem to be able to fly his aircraft straight and level for 30' seconds in cruise flight without stalling it. From the data and CVR he appeared to be struggling with pitch oscillations, which I simply wouldn't expect from a trained pilot.

I'm conjecturing, really that the change in control laws without any force feedback threw him off enough that he struggled to contain the oscillations. I'm sure that roll inputs feel very different in the coffin corner than they do low and slow and usually the computer accounts for that. With the Dash 8, for example, you can immediately feel the difference between stiff or sloppy controls. With a sidestick there's no such feedback. The good thing is that the industry learns from these things and today you will hopefully be prepared in these situations better than the AF447 was.

This actually answers the question asked: "Do Airbus pilots train for flying in Alternate Law?". All the other answers address specifics of the two crashes mentioned in the question, but not the question asked. Surprised it doesn't have the check mark it deserves.

My first sentence in my answer also answered his question. "Airbus pilots do practice in alternate law when in training. As far as I know, it's a requirement". Also another person's comment stated " Training now includes flying in alternate law and basic law in multiple flight states, including cruise". I don't see how those statements were not answering the question. Of course nowhere near as detailed as Anas Maaz own in regards to practicing to fly in alternate law since I'm not a pilot and he's a real A320/A321 pilot. His answer should have the check mark tho.

Airbus pilots do practice in alternate law when in training. As far as I know, it's a requirement. As you note, in alternate law (A320 family) and alternate law 2 for the other FBW widebody Airbuses, direct proportional commands is given to the ailerons and spoilers where roll sensitivity depends on the airspeed. Combine that with the lack of artificial force feedback to stiffen the sidestick at such high speeds with a startled pilot who's taken by surprise and is required to make an input right away to keep wings level after the aircraft starts banking (or yawing like in AZ8501 thanks to the slight rudder deflection), it's no wonder why both co pilots of AF447 and AZ8501 were overcontrolling in the roll axis. Normal law with it's roll rate, pitch rate/g-load and the flight envelope and speed protections provide mitigation for the lack of force feedback and changing speeds but it can be real problematic when you ain't in normal law and a pilot gets startled at high altitudes and is overcontrolling. A non startled Airbus pilot might be able to control the roll axis in alternate or direct law just fine but with a startled one, that mode change might cause an issue.
Although imo the autotrim in combination with the lack of artificial force feedback in the longitudinal axis were bigger problems imo. Basically it's easier to stall a FBW Airbus in alternate law (if the other pilot don't take priority with his sidestick push button) than pretty much any conventional airliner or C * U FBW equipped airliner flying right now since there's no increased artificial feel pressure at those higher speeds other than a basic centering spring that don't change force based on airspeed and with alternate law, the autotrim  will trim the trimmable horizontal stabilizer (THS) automatically to maintain nose up attitude and you don't have any out of trim force to have to trim for. The A320 family in alternate law has low speed stability where it will introduce a nose down input if the AOA is too high and the speed is getting to low but even that can be overridden by just keep pulling back on the stick with little to no challenge. With alternate law 2 in the widebodies, low speed stability is lost. All these is why I prefer C * U FBW in aircraft like the A220, 777, 787 etc over the C * FBW used in FBW Airbus and the Dassault Falcon 5x-10x aircraft. Every C * U aircraft I know of has artificial force feedback regardless of the FBW mode it's in to decrease the chance of over control at higher speeds in roll and pitch and requires manual trim to trim out the forces where the THS moves automatically afterwards, even though in normal mode at least you're trimming a reference speed and C * U is speed stable, allowing for speed cues. Airbus had to get special conditions approved from the FAA and if it wasn't for normal law with all it's protections and the use of rates in the roll and pitch axis along with g-load, FBW Airbus aircraft would not have been certified according to some pilots from a few aviation websites and an A350 captain who told me this in a YouTube comment.
All those cues and tactile feedback from C * U FBW aid in situational awareness and is not a guarantee to make a disoriented pilot not persist with nose up inputs to stall the airplane, but certainly would not have been as easy to do at higher speeds and high altitude. It's better to have those cues cause it may help the pilot get back to his senses and return the plane back to normal level flight. These could really make a difference when a pilot is confused and disoriented and has to manually hand fly, especially if there's a FBW mode change. 2 Boeing 777 captains I came across said it's very difficult to keep a continuous nose up attitude at high altitudes and airspeed to stall the plane as the artificial feel forces extremely strong and you have to consciously trim for the horizontal stabilizer to move. It won't move on it's own to help make it easy to keep a nose high attitude to stall it. Even an Airbus Captain in YouTube comments who flew the A330/340 previously (who questioned how the FBW Airbus's got certified in the first place), told me AZ8501 and AF447 might not have even stalled had the C * U FBW system been in place as it might have made Bonin thing twice about persisting with nose up commands and might have made them cease doing that, prevented the stall and thus preventing the 2 accidents entirely. I like FBW Airbus aircraft and they have a great safety record, but I definitely prefer C * U FBW it's competitor Boeing uses, especially the advanced one on the 787. Has more benefits.

Fully agree. Active force feedback is a necessity in an emergency.

...What are "C * " and "C * U"?

From how I understand it, C* is the FBW version where in regards to pitch inputs there's a gradual change from pitch rate (low speed) to g-load factor (high speed). C* FBW maintains neutral stability for the aircraft that uses it and requires an autotrim system for the horizontal stabilizer to maintain the pitch attitude the pilot sets when the stick is released. It will keep the attitude regardless of airspeed loss which is a consequence of C* FBW. This design eliminates speed cues as a result and artificial feedback isn't provided. Airbus FBW and the Dassault Falcon 5x-10 don't provide that.

C*U is the speed stability version where you're provided feedback on if the aircraft is trimmed for speed or not. The pilot trims for speed changes and the aircraft has longitudinal static stability, where the aircraft will change pitch depending on if the airspeed is getting slower or higher from it's current trimmed airspeed. The pilot trims for a certain airspeed and then the horizontal stabilizer will move to allow for it. artificial feedback is provided. These provides a cue if you're low on airspeed or flying fast. Aircraft with this FBW version fly more like a conventional aircraft.

With C*U artificial feedback is provided for not only the pitch axis, but the roll axis also on all the aircraft that has it that I know of.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Is there a preferred wind direction for a helicopter in hover?
When hovering, how does a pilot deal with wind direction? As when landing is it preferable to point the nose into the wind? Is this a primary consideration for the pilot considering that he may need to maintain visibility on something, such as during a rescue or a news operation?
And is wind direction more of a factor when hovering in or out of ground effect?

In most helicopters, the wind direction when hovering is of concern to the pilot, as is which way you turn when hovering.
The following applies to a single rotor helicopter with the tail rotor mounted on the left and the rotor turning counter-clockwise.  The opposite applies to clockwise rotating rotors and mounting the tail rotor on the left versus on the right would take more pages of text!

Hover with no wind.

As the rotor rotates counter-clockwise, the torque reaction on the fuselage wants to turn the nose to the right.  The tail rotor "pushes" the tail to the right to balance this torque.  It is by varying the angle of attack of the tail rotor that we can change the amount of thrust from the tail rotor and therefore have yaw control.
The tail rotor is no different to the main rotor, it just operates in the Y axis rather than the Z axis and is subject to vortex ring state.
The vortex from the tail rotor is to the left.  If the helicopter is yawed right, the tail moves left and the tail rotor moves into it's own vortex causing a reduction in angle of attack and therefore thrust.  If the turn is too fast, this reduction can lead to a loss of tail rotor effectiveness.
Also in a no wind hover, the vortex from the main rotor is forced downwards.
All of what follows is for a counter-clockwise rotating rotor and applies to all single rotor helicopters.

Hover with the nose into wind, but not with enough wind to generate
  effective translation lift.

This state is more or less the same as for a no wind hover and is the easiest (and therefore preferred) direction in which to hover.  Turning the aircraft has the same risk as in a no wind hover in that turning too quickly to the right can lead to loss of tail rotor effectiveness.

Hover into wind with enough wind speed to generate translational lift
  (let's say about 15 kts)

Now things start to get interesting.  If you turn to the left, the vortex from the front edge of the main rotor is blown backwards by the wind across the tail rotor and causes a decrease in the angle of attack of the tail rotor blades since the wind from the vortex is blown on to the right hand side of the tail rotor.  Therefore, a lot more left pedal is required as the helicopter turns from 0 degrees (relative to the wind) to 270 degrees.  As 270 degrees is reached and passed, the vortex is no longer interfering with the tail rotor and the angle of attack for the same pedal position is increased.  If the same pedal input is maintained, the rate of turn will increase rapidly and if not corrected, can lead to an uncontrolled spin.
If you turn to the right, the opposite happens.  More right pedal (less thrust) is required in the first quadrant of the turn from 0 degrees to 90 degrees since the vortex is blown into the face of the tail rotor causing an increase in angle of attack and therefore greater thrust.  As the nose passes 90 degrees, the angle of attack decreases and left pedal must be added to prevent a sudden acceleration in yaw.
You should be able to see that the amount of pedal required changes as the turn continues and without constant adjustment, a steady turn cannot be maintained and with enough wind, and a pilot who is not quick enough on the pedals, control can be lost.
Now let's take the turn out of the equation and consider steady hovers in wind that is not "on the nose".

Wind from the left quarter, i.e. 270 - 0 degrees. This is effectively the same condition as that first quarter of a left turn with wind from the front.

This is the difficult one.  Wind is never constant. It is always changing slightly and turbulent.  As the main rotor vortex is blowing into the tail rotor, the tail rotor is operating in a very turbulent and unpredictable environment.  Rapid and constant pedal inputs are required to maintain control.  Any deviation not caught immediately can lead to a loss of control.  It is very difficult to hover with the wind from the left quadrant and is therefore usually avoided.

Wind generally from the left (a left crosswind)

As the wind is blowing into the face of the tail rotor, its angle of attack is reduced as the airspeed through the rotor increases.  This can lead to the loss of tail rotor effectiveness or even a stall of the blades as the angle of attack exceeds the critical angle.

Wind from behind.

The primary effect here is the tendency of the fuselage and vertical stabiliser to "weather cock" the aircraft.  It naturally wants to face into wind.  If a yaw begins and is not immediately corrected, due to the effects above, the yaw can increase rapidly and suddenly.  Additionally, the wind is never constant and varies between passing to the left and to the right of the tail rotor.  Therefore, hovering downwind needs immediate and constant corrections to yaw.

Summary

Hovering into wind is very much preferred and is always chosen unless operations require something different.  Hovering with the wind from the right is the next "most" preferred position with quartering winds from the left and hovering downwind to be avoided if possible.
All of this said, it's a standard part of training to learn how to hover in these conditions and how to maintain constant rate turns.  It does require a lot of learned skill and you must be quick and accurate on the pedals with constant corrections.
Incidentally, early models of the UH-1 were particularly difficult to handle in certain wind conditions and the tail rotor was moved from the right to the left on later models to improve handling.
In a light helicopter with little power reserve, e.g. the R22, hovering in strong gusty winds is an intense part of operations.  I had some close calls when learning how to do this including a loss of control which I only recovered by pulling in power and accelerating into forward flight.  Not good on an airfield with other aircraft operating since my "low hours brain" focussed only on regaining control.  I am ashamed to say that looking around took second place but I did learn an important lesson and with hindsight, the right thing to do would have been to land and discontinue the flight as the wind got more gusty.

Only thing I'm not getting is, with ccw rotor, the thrust from the tail rotor is to the right. So why is its vortex to the right? Wouldn't vortex be opposite to the direction of thrust?

It seems like it would be extremely disconcerting if you're used to flying a ccw helicopter to switch to a cw one.

@TomMcW Sorry, you're right.  My excuse is that it was early in the morning when I wrote this and still on my first cup of coffee.  Corrected it now.  I've never flown a CW rotor but I've heard that it is difficult to switch.  There are definitely times you don't want to stick the wrong pedal in, e.g. autorotation entry.

Cool, thanks. I'm accepting your answer because of its thoroughness. Aeroalias gave a great answer too and his diagrams helped. I wish I could combine the two and accept both!

this is the key phrase of the whole answer: " it's a standard part of training to learn how to hover in these conditions and how to maintain constant rate turns"

The helicopter does not differentiate between the wind and flight. i.e. it does not matter if the helicopter is flying in a direction or the wind is coming from that direction. As with landing, visibility is the primary concern for the pilot during hover (or maintaining a position, like during a rescue).
However, it better to hover into the wind rather than other positions for aerodynamic and stability reasons.

In case of wind from left of the front ~285° to 315° (if the main rotor is rotating clockwise from the top) , the vortex from the main rotor can interfere with the tail rotor, reducing its effectiveness. This may cause an uncommanded rapid rotation, which can lead to loss of control if not corrected.

Source: dynamicflight.com

In case there is a tailwind, the helicopter vertical tail (and fuselage) may align try to align with the wind, resulting in uncommanded yaw. If not corrected with proper pedal input, this may lead to loss of control.

Source: morningtonsanfordaviation.com

In case of cross wind (from the left for main rotors rotating clockwise from top), the tail rotor may enter into a vortex ring state, especially in low speeds. This requires continual adjustment of pedals by the pilot to prevent the unsteady yaw.

Source: morningtonsanfordaviation.com
This should apply for both in and out of ground cases. It is usually preferable to orient the helicopter so that the thrust required from the tail rotor is minimized in case of a cross wind i.e. so that the cross wind is from the right in case the main rotor rotation is clockwise from the top.

this answer hardly applies to modern helis, which don't have these kinds of power issues.

The best situation when hovering is for there to be little or no wind. There is no "preferred" way for the wind to blow relative to the helicopter unless there is a certain place you want to see. However, because of their designs, helicopters will tend to "weather-cock" in the direction the wind is coming from due to the CP-CG relationship.
I found this FAA doc on helicopter handling with a bit more stuff: https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook/media/hfh_ch09.pdf

Please see my answer for a simplified explanation of why there most certainly is a "preferred way for the wind to blow". Please try to resist answering helicopter questions unless you have specific knowledge and/or experience.


text:
What are the pros and cons of scimitar propellers?
I know scimitar propellers are used to reduce noise, but what is their downside? There has to be one, considering most propellers don't use that shape.

Good question.  I'll be watching this one.  I know that they only provide marginal performance improvements, if any at all, for GA aircraft.

My guess would be that this kind of propellers is too costly relative to the performance and noise benefits. The performance increase may be noticeable only in the Mach > 0.8 region. But I don't have any real numbers to support.

The biggest benefit of a scimitar propeller is the higher operating Mach number that can be achieved. Tip sweep delays the onset of shocks on the propeller and reduces drag at high subsonic speed. The noise reduction is achieved by reducing shocks on the blade tips at the same tip Mach number - supersonic props are known to be extremely loud.
The major disadvantage is the torsional moment created by the swept back tip. This can be somewhat mitigated by sweeping the inner blade forward which results in the typical scimitar shape. Still, in-blade torsion moments mean that the blade has to be built such that the deformation in flight will result in the desired blade twist. The use of stiff carbon fiber allows to keep this pre-twisting low.
That scimitar propellers have a high blade count is a result of their high operating speed: To keep the tip Mach number low, the diameter must be limited which increases the load on the single blade. Flying fast needs a lot of thrust, so the activity factor (power absorbed by the propeller per blade area) is high. To reduce the local Mach number on the blade, both chord and blade count are increased while the relative thickness of the blade's airfoil is reduced when compared to a conventional propeller. This results in a lower operating lift coefficient on the blades and lower supervelocities which again helps to increase the operating Mach number.


text:
What is a (rough) rule of thumb for calculating fuel consumption in a modern jet airliner like Boeing 737 - 300?
I'm wondering what would be a rough rule of thumb for calculating the fuel consumption of a modern airliner like a Boeing 737 - 300.
I am not asking for anything from the plane's manual or even a rule of thumb to be used by pilots, but rather a "general idea", to be able to compare the fuel consumption of different modes of transportation.
I am citing the Boeing 737 - 300 as an example as it is a pretty common type of airplane, but feel free to answer for another type of passenger aircraft.

Related: http://aviation.stackexchange.com/questions/8652/how-can-i-calculate-the-fuel-consumption-of-an-airbus-320-200-at-various-loads/8654#8654

The 737-300 isn't particularly modern -- it first flew more than 30 years ago.

Here are some very rough values for the 737-300, that a certain airline uses to compute manual flight plans in case the computer system fails.
Climb - 2950lbs / 15mins
Cruise - 5500lbs/hr
Descent - 600lbs / 20mins
Hold - 2650 / 30mins
Alternate - 1950 / 20mins
Min arrival fuel - 6300lbs
Recomended arrival fuel - 8300lbs
These numbers were based on a 27000lbs payload (approx 130pax plus bags)
source: http://www.airliners.net/forum/viewtopic.php?t=722273
[EDIT: I've edited the answer to remove any mention of the 737-300 being one of the most common 737 variants, thanks for the constructive comments!]

I would bet my house that the 737-300 is no longer the most common 737! That thread is 16 years old now. Still interesting data though.

@Ben According to Wikipedia, there were 1113 -300s built, delivered between 1984 and 1999. Since the introduction of the Next Generation 737s (-700/800/900) in 1997, nearly four times that many -700s have been built and a good number of the Classics will probably have been retired. So, yeah, the -300 is nothing like the most common today, and Boeing is cranking out new -700s at a rate of more than one a day.

For a first approximation, use 55 to 65 g of fuel per Newton-hour or 15 to 18 g per kN of thrust per second with modern jet engines. Please do not use the static thrust at sea level, but the actual thrust at the right flight Mach number and altitude. If you lack this figure: A modern airliner needs thrust equivalent to between 1/15th (6.67%) and 1/20th (5.0%) of its weight. In cruise flight at Mach 0.82 a value of approximately 1/17th (5.9%) should be used.
Thrust-specific fuel consumption nearly doubles between the static case and cruise at Mach 0.82, and the maximum thrust of the engines is roughly proportional to air density, which is a quarter of its sea level value at cruise altitude (approx. 10 km). The best turbofans achieve 9 g per kN and second in static tests!
If you want to calculate the fuel consumed over a longer trip, use the Breguet range equation and reformulate it so you get the mass ratio between take-off ($m_1$) and landing mass ($m_2$) for a given range $R$. For jets this equation is
$$\frac{m_1}{m_2} = e^{\frac{R\cdot g\cdot b_f}{v\cdot L/D}}$$
Nomenclature:
$m\:\:\:\:$ mass in [kg]
$R\:\:\:\:$ Range in [m]
$g\:\:\:\:\:$ gravitational acceleration in [m/s²]
$b_f\:\:\:$ thrust-specific fuel consumption in [kg/Ns]
$v\:\:\:\:\:$ ground speed in [m/s]
$L/D\:$ lift-to-drag ratio

Great, very detailed answer. A little tough to do without a calculator (or at least pen and paper), though.

@Peter Kampf, you are just too smart for your own good!  Seriously though, do you think someone looking for a "rough rule of thumb" figure is going to use that formula to come up with a number in "newton hours"?!  I will spare you a downvote because I trust that this answer is substantially correct, but I really think answers should mirror the relative complexity of the question.

P.S.  The fact that I am probably using "newton hour" out of context actually confirms my point:  That even a relatively intelligent, college educated, military and commercial pilot doesn't think in these terms, much less a casual aviation enthusiast asking a very basic question.

@MichaelHall So you suggest I downgrade the answer by switching to Imperial units? And for thrust 1/17 of weight isn't rule-of-thumby enough?

Nope, I'm not suggesting anything.  Just offering an observation...

For an A330-200 operating in the region of FL200 its approx 100kgs/min or 6T per hour. Less at higher levels.  I'm an instructor on the military version of the A330 and this is our mission planning rule of thumb. Add 2 tonnes for the climb and a suitable reserve at destination and your fuel planning is done!
The same principle can be used for any large aircraft, fuel flow per minute in the cruise is a known quantity(either from published figures or the fuel flow guages). The length of time in the cruise is a known quantity so if you add on the minimum fuel required at destination plus a little bit extra to allow for the initial climb you'll have a ball park figure for fuel required at the start of the trip.
Despite the fact that you'll have a computer generated flight plan before getting airborne, this simple rule of thumb allows you to do a gross error check to spot any mistakes. When airborne the same principle applies.

Hi Nic, and welcome to Av.SE.  It's helpful to know what an answer is based on.  For instance, if you're an Airbus pilot & that's a rule of thumb that you use, adding that gives your answer credibility.  Otherwise, it amounts to "somebody on the internet said..." which as you can imagine doesn't really carry much weight.  Alternatively, if you can link to an authoritative source, that works well also.

Hello Nic, welcome to aviation.stackexchange.com. This is one data point for a different aircraft. It doesn't answer the question. It would be better to post this as a comment or to expand you answer so that it addresses the question.

The question stated 'modern airliner' which the A330 is.   It's relevant if you want a rough idea of fuel consumed with regards to flight time.

Upvoted, the question mentions "I am citing the Boeing 737 - 300 as an example as it is a pretty common type of airplane, but feel free to answer for another type of passenger aircraft. " so this is a good answer to the question.


text:
How does one "emergency extend" the right main landing gear of a B-52 Stratofortress?
In an April 11, 2024 video an embedded CNN crew documents a 33 hour B-52 Stratofortress round trip flight between Barksdale Air Force Base in Louisiana "flying near Russia, China, North Korea, and back."
At one point Engine #5 had an issue and was shut down.
Near the end of the video after 05:00 there is an exchange on audio, shown in captions:

One gear not down?
Affirm. Right main gear is not down.
Go ahead and emergency extend it.

Question: How does one "emergency extend" the right main landing gear of a B-52 Stratofortress?
I'm probably remembering some old black-and-white WWII movie seen on Saturday morning TV when I was young, but I'm imagining the flight engineer going to the back of the plane, lifting up a floorboard and revealing a big hand crank and struggling to extend the landing gear manually. Finally a mechanical indicator in the cockpit pops up confirming successful extension and lock (or not, enhancing the drama of the movie?)
I have a hunch that modern B-52's have a more automated and powered procedure for emergency extension.

click for full size

Many planes have/had an emergency landing gear extension mode that just releases the locks and lets gravity do the rest of the work.

After running through the usual series of checks (cycling the circuit breakers, etc), one takes the Landing Gear Emergency switch for the gear in question to EXTEND.
Reference - B52 Pilot's Manual page 367 (pdf page number)
Note: This manual is for the C/D model B52s, but my vague memory from working on the G/H model simulators in the 80s is that it worked the same way there.
Here is a schematic of the landing gear system showing the switches and the backup hydraulic system connections actuated by them.

Reference: The B-52 Stratofortress: A Case Study (partial document) (pdf page 10)
This document covers the H model aircraft so I'm more confident the answer applies now.

I see, it looks like it activates a backup/redundant hydraulic circuit - there's no manual cranking involved.

@uhoh If you want fun stuff under the floorboards, see the 737. For manual cranking a big handle, that's the A320.


text:
Which of the major airliners are bleedless?
Bleed air is compressed air taken from a jet engine, and is mainly used to provide pressure for the aircraft cabin.
Fume and smoke events, due to contaminated bleed air, are regularly reported on AVherald.com, including a 2015 event where both pilots were taken ill and the captain later died.
Most airliners in service today use bleed air, a noticable exception is the Boeing 787.
Which other airliners use bleedless systems to provide cabin air?  I'm looking for a list of aircraft models not just the Boeing 787.

The 787. And a number of combat aircraft.

@sophit  It only mentions the 787.  I'd like to know if bleedless air is used by any of the Embraer, ATRs, CRJs, Comac, Tupolevs etc. etc.

It's very unlikely to find bleedless ECS and Anti-ice on other airliner designs simply because doing it on the 787 required a massive effort to create the electrical support system to make it work, and it's safe to say there aren't any other takers.
Massive generators with the capacity to supply a town, to provide electric anti-ice and cabin heating, and to drive pressurization compressors. And to back that up, Boeing was forced to use Lithium Ion batteries to provide reserve electrical capacity that could not be met with NiCads. This was all a very radical system.
Nobody else has done this on a Western Civilian airliner that I'm aware of and probably never will after the L-ion battery fire fiasco that grounded the 787 fleet and prompted an AD to require L-ion fireproof enclosures.

Apparently the Comac C919 is also not bleedless https://www.liebherr.com/en/gbr/latest-news/news-press-releases/detail/liebherr-aerospace-delivers-first-bleed-air-system-for-the-c919-engine-test-bench.html

The C919 was developed with a lot of technology transfer from Bombardier as part of the deal to the C Series fuselage.

According to Wikipedia, Comac is developing an Chinese designed engine to replace the CFM.  I'm guessing they won't do anything as  radical as changing from bleed air to bleedless for a like-for-like swapout though.  The C929 hasn't flown yet, I haven't looked at the bleedless potential of all the possible engines that that could use, but if, as yous say, the 787 experience was so bad, I doubt the C929 will go down that route.


text:
Why use different airfoils on the same wing of an aircraft?
Why different airfoils are used in different portions of the same wing of an aircraft, for example Boeing 737 wing?
Boeing Commercial Airplane Company model 737 airfoils' dat files from here:
https://m-selig.ae.illinois.edu/ads/coord_database.html
BOEING 737 ROOT AIRFOIL (b737a-il),
BOEING 737 MIDSPAN AIRFOIL (b737b-il),
BOEING 737 MIDSPAN AIRFOIL (b737c-il) and
BOEING 737 OUTBOARD AIRFOIL (b737d-il)

Because the wind tunnel has determined what the best airfoil is, at this particular span, over the whole operational range of flight:

Stall speed: docile pre-stall behaviour with outboard wing stalling last to retain roll control.
Landing speed with flaps & slats out: high stall angle, high $C_L$
Climb speed: low drag in high lift conditions.
Cruise speed in clean configuration: low drag, low pitching moment, high drag/lift divergence speeds.

The design goal is the performance of the total wing, wing profile is one of the parameters that can be used to optimise performance, minimise drag, provide good structural characteristics, and provide a large volume for the on-board fuel.

For the wing root, the aerofoil must have a good cantilever ratio (thickness) for torsional and bending stiffness.
For the wing inboard section with flaps extended, aerofoil drag must be low in high lift conditions.
For the wing tip, the aerofoil must have a high maximum $C_L$ and gradual stall characteristics)
For all wing sections, the aerofoil must have a sufficiently high critical Mach number.
For all wing sections, pitching moment coefficient should be low to moderate, to avoid high trim drag, and high torsion at high dynamic pressure.
For all wing sections, aerofoils must not be extremely sensitive to manufacturing tolerances, dirt/bug build-up etc.

So different bits of wing have different design characteristics. The whole wing is a gradually blending unit of different aerofoils.

Additionally the outer portions of the wings can experience a twisting moment. Some airfoils may be better than others in dealing with this

@TomMcW Good point, added in.

For the same reason the wing root and wing tip don't always have the same incidence angles: you usually want the wing tip to be the last part of the wing that stalls in high angles of attack.
This is the reason that in some (most? all?) aircraft the wing tip has a lower incidence angle than the wing root.
I think this property is best visible on the F-18 (no, it's not an F-16 as the file name suggests):
http://duotechservices.com/wp-content/uploads/2015/04/f-16-The-AIM-9-Sidewinder.jpg
You can see the Sidewinder missile on the wing tip launch rail is actually pointing downward (=the F-18's wing tip has a negative angle of incidence).

The question is not about incidence angle variation, but shape variation.


text:
How is a seaplane steered on water?
Steering a seaplane on water appears to be a little more 'interesting' than steering a land aircraft, at least according to the FAA Practical Test Standards for single-engine seaplanes (ASES), which requires the pilot to:

[use] flight controls, flaps, doors, water rudder, and power correctly
  so as to follow the desired course while sailing.

So how does it all work? Why/when/how do you use doors and flaps to steer? Are the water rudders connected to the pedals (like brakes on a land aircraft) or is there a separate steering control? What about braking: is there any, or do you just drift gradually to a stop?

Seaplanes mostly have water rudders that are used at lower speeds and are retractable (actually, they tilt out of the water flow).  At higher speeds, like takeoff runs and landings, high speed taxis, they are retracted and the primary means of steering is the plane's rudder.  Water rudders are connected to the regular rudder pedals.
Airlerons can also be used to steer a seaplane on the water by creating differential lift and drag (since most ailerons don't deflect symmetrically).
Doors can be used to steer by opening one or the other, creating drag on one side or the other.  I have seen a C-150 pilot do this in the air, as well.  Flaps, which create symmetrical drag, can be used to assist in slowing down but they're not useful for steering.
Seaplanes have no brakes other than the drag of the floats in the water, so a seaplane pilot won't approach a dock at a perpendicular angle.  Instead, they will land along side a dock and then jump out onto a float and then to the dock, rope in hand.
Seaplane pilots also carry oars, which allow them, yes, to paddle, steer, and stop the plane.
So seaplane pilots have a number of tools at their disposal to steer the plane on the water. You can also read through the FAA Handbook for seaplane operations.
It does require some coordination!

FWIW I agree with the edits.

was this a floatplane: "I have seen a C-150 pilot do this in the air, as well"? if not, then it should be removed or put in parenthesis and edited to say: "I have seen a non-floatplane C-150 pilot do this in the air, as well"

I do not believe the post needs to be modified.  Yes, the C-150 that I referred to is not a seaplane.  The point illustrates that changing the aerodynamics of a plane in motion provides the pilot with additional ways to maneuver is not related to flying or moving through the water.  Why limit the meaning to land or water when in fact it works in both "flight" regimes?

"Seaplanes have no brakes other than the drag of the floats in the water" - they don't have reversible propellers?

In addition to steering a seaplane when under power, you can 'sail' a seaplane when the engine is shut down. All you need is air movement from wind or on a river from the current taking the aircraft downstream.
This is a good illustration:

Image Source
I don't think you'd want to approach a dock at a perpendicular angle doing this, but I used to often sail an old J-3 on floats to a perpendicular beaching (make sure the water rudder is up).
As an example let's say there is no wind, but you're on a river flowing west, and you want to reach the southern shore. The current is taking you downstream, so you have an effective west wind. When you shut down the engine, the aircraft will weathervane to face west. The rule of thumb is to use the air rudder and the ailerons to point the tail away from the bank you want to reach. Thus left rudder, left aileron down, right aileron up. You will move toward that bank of the river on the side which you have created the drag.
It's a lot of fun. You can also use this technique at idle power in a strong wind for more control as well as to get yourself out of trouble.
Hope I got this right, it's been 45 years since I've done it.


text:
Why are straight-in approaches dangerous at uncontrolled airfields?
Why are straight-in approaches ill-advised at uncontrolled airfields? My understanding is that the recommendation is to do a standard overhead join where possible, if there is no radio service.
But at an airfield I fly at, a straight in, long final feels safer because:

I can see straight ahead of me with a wide view, I would be able to see if someone was either ahead of me or turning base / final
Overhead joins add the risk of not seeing someone below you when descending, coming straight in mitigates that, especially if you come in slightly low

Interested in what I may be missing here.

This is very UK specific. I can see this making no sense at all to Americans, for example.

@Jamiec are overhead joins UK specific?

Im not 100% sure, I have flown a fair amount in the US and it wasn't a thing there. My flying in mainland europe is fairly limited, so I can't be sure.

Straight in approaches happen in the US, and cause accidents:  https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/105763/pdf

@AShelly arguably, that crash was ADSB-related. We'll never know for sure, but reading about the 152's behavior it's consistent with a panicked response stemming from an ADSB traffic alert (“Going around, because you are coming at me pretty quick man.”). So the straight-in might not have been the causal factor, it might have been the (mis)reliance on an ATC airspace management tool for collision avoidance.

Why are straight in approaches dangerous at uncontrolled airfields?

They're not, inherently. Everyone doing their own thing instead of following convention might be, though!

The CAA safety sense leaflet on "strip flying" feels relevant to your question, and has this to say (Emphasis theirs!):

If cloud base and airspace
permit, an ‘overhead join’
will allow you to observe
the windsock and any
obstructions on the strip.
Be alert for wildlife or livestock.
Keep circuits compact and avoid
overflying any noise sensitive
areas.
Even if the strip seems quiet,
be alert for other aircraft. It
is not unknown for passing
aircraft to use strips for practice
forced landings or even conduct
ad hoc ‘beat-ups’.

If you're joining on a 4+ mile final, spotting a couple of sheep or pedestrians/dogwalkers is difficult.
The main reason usually cited is spotting the windsock, but if you're 100% sure of the wind direction at ground level, this seems less of an issue.
The Skyway code also has this to say:

The ‘overhead’ join is the recommended
joining procedure for uncontrolled aerodromes.
It allows you to observe traffic in the circuit
prior to entry.

While I don't disagree that you might have better visibility coming in long final, if everyone follows the same procedure, then everyone else knows what to expect.

I think one of the big problems people have with someone flying a straight-in approach is that they often ignore the traffic in the pattern and just barge in with a "I'm on final, get put of my way." attitude.

In the US, FAA AC 90-66C speaks to this topic, just FYI.

@AeroAndy Oh interesting, I didnt think you guys had an concept whatsoever of an "overhead join". Every bit of flying ive done in the US has been join 45 to downwind.

In addition to being a way to observe the runway conditions, doing an overhead join ensures that the aircraft has some altitude margin while being just above the runway: this way, if things go wrong, a shortened approach is always a solution.
This may not be considered for controlled airfields, where traffic separation is a priority.
This is the recommended approach for gliders, where the pilot really needs to check for local winds; but is also useful for motored aircraft, where an engine failure is always possible and would be a bad situation when engaged in a long final already on 3 degrees path.

A friend of mine narrowly avoided a midair collision right after takeoff when another small plane was executing a straight-in final on the same runway. After landing, he never flew again.
Flying a pattern approach forces the pilot to identify and join into existing traffic which requires radio communication and situational awareness.


text:
Design and performance of Bi-Planar Rotors or Propellers
I have been doing a bit of a deep dive on coaxial rotors and propellers recently and ended up reading into the NASA Puffin vtol concept. I found out that it was supposed to have a "bi-planar" rotor, that is to say two sets of blades rotating in two planes, and rotating in the same direction. (IE: not opposite like a coaxial). They claimed it was more efficient than a coaxial rotor and less noisy.
Is that true? Is there any literature on how this bi-planar rotor (or propellers) works?

Photos suggest the Puffin had what looks like just two coaxial rotors.  Where does NASA say otherwise?  Was each rotor actually four-blade instead?  Or, within each coaxial rotor, did one blade-pair really go in the same direction as the other blade-pair?

coaxial   just means "on the same axis" - it does NOT mean contra-rotating...

BLUF: a stacked (i.e., coaxial rotors that are co-rotational) rotor system is generally more efficient (consensus seems to sit on the order of 5-10% in general cases) and quieter than an isolated rotor of the same size and solidity that are trying to produce the same amount of thrust. Two isolated rotors will always be more efficient in producing the same thrust as two coaxial/stacked/tandem rotors since they will not have to face performance degradation due to rotor-rotor interactions.
Most stacked rotor references in the below are drawn from a 2019 NASA study entitled An Exploration of the Performance and Acoustic Characteristics of UAV-Scale Stacked Rotor Configurations, a (paywall) 2023 study entitled Exploration of stacked rotor designs for aerodynamics in hover, and Investigation of the Flow Fields of Coaxial Stacked and Counter-Rotating Rotors Using PIV Measurements and URANS Simulations from 2021. This is by no means a comprehensive summary of stacked rotor research, but the citations in those should help.
Note on terminology: in the below, coaxial will always refer to coaxial rotors spinning in opposite directions, and stacked will always refer to coaxial rotors that are co-rotational.
Complete answer:
The driving factor for coaxial rotors is, usually, a way to resolve a competition between footprint and performance. Thinking of the Kamov Ka-50, its coaxial rotor enables higher maneuverability through a reduction in moment of inertia (think of the transmission and rotor weight that would otherwise hang out on the tail and tail boom). This also means that it has a smaller footprint, making a derivative of it, the Ka-52, of interest for naval applications. For military applications, performance is, generally, the driver. This is, coincidentially, why you don't see a bunch of civilian coaxial helicopters...they require more fuel to operate, and cost is the primary driver for that market. Drones break the curve with that a little, because electrical power is a little cheaper than traditional fuels on a joule for joule basis, and volume (and weight) are commodities when it comes to operating them, hence why it makes sense that interest in coaxial or stacked systems is a hot topic for them.
That said, stacked designs are not new. The AH-64 Apache (first flight Sept 30, 1975) actually has a stacked tail rotor (see below) with a small azimuthal offset. From what you can see of the individual blades, you can verify that this is not a coaxial rotor system, but a stacked one.

While this was initially done to improve mechanical access, it was found to reduce the 4/rev vibrations of the rotor (i.e., impulsive forcing from each blade passing over the tail boom, which would normally occur with 90 degrees of separation for a 4-bladed rotor) and, ultimately, noise. In the 2023 study mentioned earlier, it was found that, for the model rotor tested, depending on the aziumuth offset between rotors, the peak-to-peak thrust variations of the stacked rotor system could be smoothed out significantly, decreasing changes in thrust over a rotation by up to 60%. This indicates a dramatic decrease in vibration and noise.
This was confirmed by the 2019 NASA study cited earlier. The study went on to measure the power required and noised produced by a model rotor as it produced a certain amount of thrust, both in stacked and side-by-side configurations. This, as shown below, indicated a small (4-5%) increase in efficiency (as measured by power loading) as opposed to a pair of co-planar rotors (dZ/R = 0 refers to the co-planar case).

It is interesting to note that coaxial rotors have been tested with a similar idea. In a 2013 study, it was found that a model coaxial rotor, when producing the same thrust as a single, isolated rotor, have a greater efficiency (almost 10% greater). This is to be expected, given swirl recovery at the least. However, when compared to a pair of side-by-side rotors, the coaxial configuration generated almost 20% more induced drag, requiring almost 20% greater power for a given operating point. This is suspected to be principally due to strong blade/blade interactions when they pass each other, generating a strong drop in pressure between the blades when they pass and decreasing the amount of lift produced by the lower rotor (and increasing vibrations/etc.). See below for an illustration.

All that said, the 2019 study was primarily experimental, and the authors note several issues with a companion numerical analysis they were doing. The primary issue is that it required a proscribed inflow and a proscribed wake geometry, and the simulation results did not match experimental data. Other authors (see 2023 study) postulate, through, that there is a beneficial blade-vortex interaction, generating upwash on the lower blades. Their study as well as this one from the 2021 Vertical Flight Society forum indicates a strong relationship between thrust production/power requirements in a stacked rotor system and the placement of the lower rotor with respect to the tip vortex of the upper blade. This vortex can increase the inflow (and thus the power loading) around the tip of the lower blade, generally increasing aerodynamic efficiency of the lower rotor. This can be seen in the below comparison of thrust production with and without strong blade vortex interaction in a stacked rotor system as compared to isolated and coaxial rotors from the 2023 study that I cited earlier. Aerodynamic efficiency appeared to maximize in their study at azimuth offsets of  5-10 degrees when the axial separation between rotors was 0.2 diameters, 20-30 degrees at 0.3 diameters, and between 30-90 degrees at 0.4-0.5 diameters of separation.

However, there is no free lunch. By co-axially positioning two rotors, and in close proximity, you get two simultaneous effects. There is some effect of the lower rotor on the upper, although the upper rotor generally tracks with the inflow velocity of an isolated two-bladed rotor (i.e., the same rotor that is being stacked), as shown below (from 2023 study). The lower rotor does pull additional inflow through the upper rotor, which is especially noticeable near the blade tips. By contrast, however, the lower rotor is forced to ingest the upper rotor's wake, resulting in a dramatically increased inflow velocity as compared to the isolated case. Whereas inflow velocity varies inversely with power loading, this decreases the efficiency of the lower rotor substantially.

The pair of side-by-side rotors (think Focke-Achgelis 61 or 223, or Mil-12) will involve a larger volume of air into the production of lift, accelerating it by less. Like with a wing of higher span, this will be more efficient than stacking the rotors, regardless of their direction of rotation. Of course, losses from interaction will come on top. –

That's a great way to explain it. I will add!

This technology has been called numerous things over the years.
Hamilton Standard investigated 'Variable Camber Propellers'.
There was some old research done by Sikorski that called it 'Variable Geometry Rotors'.
Most current literature in this area calls it 'Stacked co-rotating rotors'.
Search for those key words and you will find many papers investigating the idea in different ways.
Like many technologies, it can be applied in different ways to different effects.
For example, if it is applied in a way that maximizes the noise reduction, it might result in an efficiency loss or a reduction in maximum thrust available.  Conversely, if it is applied in a way that maximizes the thrust attainable, it may result in a noisy design.
Note, the two blade pairs are not always kept at 90 deg separation.


text:
Is center tank fuel required to start the 737-800 engines?
I have been doing some simulation flights with the 737-800 and when there is no fuel in the center tank, the engines won't start. Is it like that in real life?

So you did them in a simulator? In which simulator?

In X-Plane 11, Zibo's 737-800 mod, I can fly with no fuel in the center tank. :-)

No, it is perfectly normal to operate without any fuel in the center tank. Until your fuel load is over about 17,000 lbs, the center tank should be empty. That’s enough for about 2 hours of flying, plus reserves.
If your simulator requires fuel in the center tank in order to start the engines, it isn’t accurately replicating the real aircraft.

If you let the center tank go completely dry you’ll get an EMDP malfunction along with an overheat warning.  (See ELEC 2 & ELEC 1 hydraulic pump switches on the overhead panel).
The EMDPs are cooled using what are best described as fuel cooled heat exchangers. The temperature sensor for the EMDPs is monitoring the temperature of the case drain fluid, which reflects the source of greatest heat output on a normal pump. There are a number of events that will cause the EMDP to overheat:

Every time you cycle the pump from OFF to ON, it increases the pump temperature briefly. Cycling the pump off/on repeatedly without allowing adequate time to cool will cause the overheat indication.
Operating the EMDP without sufficient fuel in the main tanks (from memory: 1,675lbs) will also cause overheating because the fuel quantity is insufficient to provide an adequate cooling mass to the heat exchanger.
An internal failure of the pump might also cause the overheat light.

In the 1st paragraph, you say "center" tank & that's incorrect - fuel in the center tank doesn't cool the electric hydraulic pumps. Later in the answer you discuss the role of the main tanks cooling those pumps, and that's correct. You may want to edit that first paragraph.


text:
Can someone please help me identify this aircraft?
I apologize for the image quality as it is a picture of an image on a television screen.  I was watching an episode of American pickers at the time and the model of the plane was not discussed.  Can anyone identify this aircraft?  I am not sure I have seen a configuration like it before.

It looks a lot like a Curtiss Model D to me.